Resonant actuators having the constitution shown in FIG. 7 (A) have been known. The resonant actuator 200 shown in FIG. 7 (A) is constituted by a piezoelectric ceramic material 202 formed in the shape of a square bar, with electrodes 204a, 204b provided on both end faces of the material. The resonant actuator 200 is polarized in the direction of arrow a and constituted in such a way that an AC electric field is applied to the electrodes 204a, 204b to drive the actuator at and near a resonance frequency so as to obtain vertical vibration in the direction of arrow b which is the same as the polarization direction a. Normally an AC electric field is applied to the electrodes 204a, 204b by connecting conductive wires 208a, 208b connected to an AC power supply, to the electrodes 204a, 204b, via spring terminals 206a, 206b, or by directly connecting the electrodes 204a, 204b to the conductive wires 208a, 208b. 
Also, International Patent Laid-open No. 2008/090758 (Patent Literature 1) proposes the constitution shown in FIG. 7 (B) by considering possible inhibition, by the spring terminals 206a, 206b and conductive wires 208a, 208b, of the vibration of the resonant actuator 200. The constitution of the resonant actuator 250 shown in FIG. 7 (B) is to suppress inhibition of vibration by using lead conductors 258a, 258b to lead out electrodes 254a, 254b, provided on both end faces of a piezoelectric ceramic material 252 of square bar shape, to the centers of the side faces of the piezoelectric ceramic material 252, while pressing connection electrodes 256a, 256b using spring terminals 260a, 260b. 
This type of resonant actuator is considered to be normally displaced, in the vibration direction, by an amount proportional to the piezoelectric constant d. In the field of ceramic materials for resonant actuators, therefore, active research and development efforts have been underway to obtain piezoelectric materials based on Pb(Zr,Ti)O3 (lead zirconate titanate (hereinafter referred to as “PZT”)) having a high piezoelectric constant. For example, “Atsuden Zairyo no Shin Tenkai (New Development of Piezoelectric Materials)” by Sadayuki Takahashi, TIC Co., Ltd., New Ceramics Vol. 11, No. 8 (1998), pp. 29-34 (Non-patent Literature 1) describes the large amplitude characteristics of piezoelectric ceramics used for piezoelectric actuators and other power devices, utilizing the large amplitude elastic vibration of piezoelectric ceramics. Non-patent Literature 1 reports that, while theoretically the vibration speed (=vibration amplitude×frequency) changes in proportion to the applied electric field E, driving PZT piezoelectric ceramics at a resonance frequency will cause the vibration speed to gradually drop to and eventually below the theoretical value once the electric field intensity exceeds a certain level.
Also, “Haipawaa Zairyo no Hyoka (Evaluation of High-power Materials)” by Sadayuki
Takahashi, TIC Co., Ltd., New Ceramics (1995), No. 6, pp. 17-21 (Non-patent Literature 2) reports that driving PZT piezoelectric ceramics at a resonance frequency will cause the resonance frequency fr and mechanical quality coefficient to drop once the vibration level exceeds a certain value. In addition, it is known that a resonant actuator using conventional PZT piezoelectric ceramics will see its resonance frequency and mechanical quality coefficient drop as the vibration speed rises. When AC voltages of 0.05 V, 0.11 V, 0.14 V, 0.20 V, 0.26 V and 0.33 V were actually applied to a resonant actuator of a PZT piezoelectric ceramic and the drive frequency was swept from above to below, and from below to above, a resonance frequency, the resonance frequency representing the maximum vibration speed was moved to a low frequency side as the amplitude of the AC electric field increased, as shown in FIG. 7 (C). The vibration speed did not return even when the frequency was lowered, and a hysteresis was observed.
This suggests a need for a feedback circuit to follow changes in the resonance frequency fr. In the field of piezoelectric actuators, etc., where high-power materials of high vibration levels are required, evaluation methods for piezoelectric property as well as relationships of compositions of PZT piezoelectric ceramics on one hand, and vibration level characteristics and other high-power characteristics on the other, are reported. To solve this problem, International Patent Laid-open No. 2007/083475 (Patent Literature 2) reports a resonant actuator constituted by a ceramic offering high mechanical quality coefficient, being highly temperature stable, and containing an oriented bismuth layered compound, by arguing the relationship of “Vibration speed α (Elastic constant)½×Piezoelectric constant×Mechanical quality coefficient×Electric field.”